Structure and Bonding in Organolithium Compounds

There is considerable controversy regarding the degree of covalent character in a carbon-lithium bond15-1S). An uncritical comparison of electronegativities does indicate a high degree of ionic character as do extended Hiickei molecular orbital 

The relative reactivities of alkyllithiums as polymerization initiators38,39) are intimately linked to their degree of association as shown below with the average degree of association in hydrocarbon solution, where known, indicated in parentheses  

Relative Reactivity of Alkyllithium Initiators Styrene menthyllithium(2) sec-BuLi(4) /-PrLi(4-6)  

It is clear that, in general, the less associated alkyllithiums are more reactive as initiators than the more highly associated species. The effect of solvent on initiator reactivity is also consistent with the importance of association phenomena. Aromatic solvents, which tend to decrease the average degree of association and promote dissociation processes of aggregates, are reported to lead to initiation rates which are two to three powers of ten faster than in aliphatic solvents 30,40). 

Dienes menthyllithium(2) icc-BuLi(4) j-PrLi(4-6) r-BuLi(4) i-BuLi n-BuLi(6). 

---